The present invention relates to a battery pack having a plurality of batteries, and a power tool using the battery pack.
As the batteries for use in power tools, generally, the nickel-cadmium secondary batteries are used. Recently, however, nickel-hydrogen secondary batteries have come to be used. In the high power trend of power tools, the battery pack used in the power tools is required to withstand a high voltage and a strong electric discharge, and therefore the number of secondary batteries used in one battery pack is increasing rapidly.
Conventionally, this kind of battery pack is constituted as shown in FIG. 5, in which a plurality of cylindrical secondary batteries 1 are stacked up in layers, and electrodes of each battery 1 are connected electrically in series or parallel by means of terminals.
Secondary batteries such as closed type nickel-cadmium secondary battery and nickel-hydrogen secondary battery are elevated in temperature due to Joule heat generated at the time of charging and discharging and the reaction heat due to gas absorption reaction. Therefore, when multiple of cells are disposed in a specified space as in the conventional battery pack, the battery temperature may exceed 80xc2x0 C. (The exothermic temperature differs with the charging current or discharging characteristic of the battery.) When a plurality of batteries are stacked up in layers, the battery position in the center of the plurality of batteries is poor in heat release performance because it is surrounded by other batteries, and the temperature rise of the battery in the central position is extremely high as compared with other batteries.
Besides, the heat is likely to be transferred upward, and therefore the temperature is higher in the batteries in the upper position as compared with the batteries in the lower position of the stack of batteries. Thus, the battery temperature rises and the individual stacked batteries have mutually different temperatures. Generally, in the alkaline secondary battery, at high temperature, the charging characteristic is lowered and the charge-discharge cycle life characteristic deteriorates.
To prevent temperature rise of battery, a method of placing metal plates between cells and release heat from one end is disclosed, for example, in Japanese Laid-open Patent No. 7-14616. Japanese Laid-open Patent No. 6-223804 discloses a method of installing a cooling plate only in the battery placed in the center of the stacked batteries.
When the battery pack having stacked batteries is used in a power tool, the battery pack is put in a discharge compartment in the main body of the power tool. In this structure, the electric power is supplied when loaded, and after discharge, it is put in a charge compartment of a charger, and is charged.
In such structure, however, the battery positioned in the center among the stacked batteries is poor in heat release because it is surrounded by other batteries. As a result, the central battery is elevated in temperature due to heat generation during charging, and the charging efficiency in the central battery is lowered. When the batteries are discharged, since the motor of the power tool is driven, a large current flows. As the large current flows, the battery temperature is further elevated.
It is hence an object of the invention to present a cylindrical secondary battery pack for suppressing lowering of charging characteristic and deterioration of life by lowering the temperature rise of batteries due to heat generation. It is also an object to present a power tool using such battery pack.
The battery pack of the invention comprises:
a plurality of stacked batteries,
a case enclosing the plurality of batteries,
a heat collector placed near the battery position in the center of the plurality of batteries, and
a heat radiator coupled to the heat collector, and placed at one side of the case.
Preferably, the plurality of batteries are a plurality of secondary batteries.
Preferably, the battery pack further comprises a first heat transfer unit placed by connecting the heat collector and heat radiator.
Preferably, the battery pack further comprises a heat transfer plate placed at other side of the case, and a second heat transfer unit placed between the heat collector and the heat transfer plate.
Preferably, each battery of the plurality of secondary batteries has a cylindrical shape.
Preferably, the plurality of cylindrical batteries are stacked up in the state that straw bags are stacked.
In this constitution, heat generation of the battery pack due to charging and discharging of batteries is suppressed. During use, deterioration of discharge characteristic is prevented, and a secondary battery pack having an excellent discharge characteristic is obtained. Moreover, a secondary battery pack having a high power is obtained.
The power tool of the invention comprises:
a housing,
a power unit placed in the housing,
a junction placed in the housing, and
an external heat collecting plate placed in the junction, for contacting and connecting a battery pack as the driving source.
Preferably, the power tool further comprises the battery pack placed in contact with the external heat collecting plate.
Preferably, the power tool further comprises:
an opening formed in the housing, and
an external heat radiator placed near the opening in the housing, in which the heat in the external heat radiator is released through the opening.
Preferably, the power tool further comprises:
an opening formed in the housing,
an external heat radiator placed near the opening in the housing, and
an external heat transfer unit placed between the external heat collecting plate and external heat radiator, in which the heat generated in the battery pack is released from the opening through the external heat collecting plate, external heat transfer unit and external heat radiator.
In this constitution, deterioration of driving force during use is prevented, and a power tool capable of maintaining a specified driving force continuously is obtained.